MiRNAs are tiny non-translated RNAs of about 22 nucleotides that are capable of modulating translation. We would like to understand their role in neuronal function. This proposal will verify a series of findings that must be true to satisfy a general framework hypothesis that miRNAs are captured by a synAPtic tag to implement enduring long term potentiation. To satisfy this hypothesis we need to know whether miRNAs are (a) transcriptionally regulated and if so whether they are transcriptionally regulated following synaptic activation, (b) whether miRNAs are translocated to dendrites, (c) whether suppression of miRNAs can interfere with any of the facets of plasticity. Each of these requirements will be tested in this proposal; however additional experiments, which fall beyond the scope of this proposal, will be necessary to verify the general hypothesis including ultimately direct physiologic measurements of LTP in the context of miRNA suppression. For the studies proposed here we first need to identify specific miRNAs involved in plasticity. In the preliminary data we present our discovery of a large number of brain miRNAs. To accomplish our goals we have built the first miRNA arrays and this technological innovation will allow us to visualize the regulation of all known miRNAs in a single experiment. We will validate these arrays and use them for determining specific miRNA expression profiles related to the stage of cellular maturation as neurons undergo polarity acquisition. We will attempt to identify those miRNAs localized to dendrites. We will analyze miRNA transcriptional profiles following receptor stimulation and following direct activation of the CRE promoter. Finally we will build upon our recent technological innovation, which has extended the RNAi technique to the suppression of specific transcripts in neuronal culture. We will determine the effects of global suppression of miRNA maturation by RNAi interference targeted to Dicer. In particular, impaired neuronal polarity acquisition, synaptogenesis, and translation of specific mRNAs are possible phenotypic outcomes of Dicer suppression.